The manufacture of melt-spun elastomeric polyurethane threads which consist of at least 85 weight percent of segmented polyurethanes is known. Such threads, however, were practically useless because of the tackiness after spinning. A solution of this problem was achieved by a chemical alteration of the polyurethane.
Such a process, in which the tackiness of extruded endless threads was reduced by chemical means, is known (DE-A-22 04 470).
In this known process, polyimides are built into the polymer chain or are added to the polymer melt. The endless threads produced with the polymer thus obtained must be wound on spools at very low speeds, and must then be stretched in a second operation. No information is given about the properties of the threads thus obtained.
Disregarding the fact that the addition of additives produces a reduction of the molecular weight which at the same time brings about a decrease in the melt viscosity, which in turn adversely influences the elastic properties, the tear enlongation and the strength of a resulting yarn, a productivity achieved with full winding speeds of about 160 m/min is inadequate and uneconomical.
From DE-A-19 44 507 a multi--step process is known which reduces the tackiness of elastomeric polyurethane threads in the spinning process. In this process the material is melt-extruded in a first step, the thread thus obtained is strengthened by quenching, and in a second step it is stretched by at least 30%, and in a further step it is relaxed by at least 50% prior to winding on the spool. In a theoretical discussion it is stated that in the case of a small relaxation ratio, that is, at a more accelerated spool winding speed, the tackiness of the thread increases.
In accordance with that process the melt-spun thread is stretched and subsequently again relaxed. The course of this process suggests that the finished, completely cooled elastomer thread is already present on the take off spool. This thread shows the typical properties of polyurethane elastomers; it can no longer be stretched in the strictest sense, but based on its high elasticity it can be strongly deformed, where this deformation is reversible. The deformation process in accordance with DE-A-1944 507 has no great effect on the properties of the thread.
All attempts to manufacture elastomeric polyurethane threads under economical conditions, that is, at higher spinning speeds by spinning from a melt, have heretofore failed because the extruded filaments stick together as soon as the spinning speed is increased above a certain value.